Conventional liquid developers for electrophotography generally comprise a liquid medium having high insulating properties and low dielectric constants, such as petroleum aliphatic hydrocarbons, having dispersed therein organic or inorganic pigments or dyes, e.g., carbon black, nitrosine, phthalocyanine blue, etc., and natural or synthetic resins, e.g., alkyd resins, acrylic resins, rosine, synthetic rubbers, etc., and further containing therein a polarity regulator, such as metallic soaps, lecithin, linseed oil, higher fatty acids, polymers containing vinylpyrrolidone, etc. In these liquid developers, resins are dispersed as insoluble latex particles having a diameter of from several to several hundred micro-meters. However, since bonding between soluble resins for dispersion stability or polarity regulators and the insoluble latex particles is insufficient in such conventional developers; thus, the soluble resins and regulators are apt to diffuse into the solution. As a result, the soluble resins for dispersion stability become separated from the insoluble latex particles during long-term preservation or repeated use, causing sedimentation, aggregation or accumulation of the particles or adversely affecting polarity. These aggregated or accumulated particles are difficult to redisperse in the medium, and they adhere to various parts of a developing machine, which leads to stains on image areas or causes problems with respect to operation of developing machines, such as clogging the feed pump, and the like.
In order to overcome these disadvantages, it has been proposed to chemically bind the soluble resins for dispersion stability and the insoluble latex particles as disclosed, e.g., in U.S. Pat. No. 3,990,980. However, this technique is very likely to produce particles having two or more peaks in a size distribution curve or a broad distribution, thus the inherent difficulty in obtaining particles having a controlled size occurs. In addition, the resulting developer has poor reproducibility.
Japanese patent application (OPI) No. 63855/86 (the term "OPI" as used herein means an "unexamined published application") discloses means proposed to eliminate the problems associated with the above-mentioned U.S. Patent. However, although the particles prepared in accordance with this disclosure have satisfactory mono-dispersibility, redispersibility, and stability as to preservation, it has still been demanded to further improve printing durability, and particularly on large-sized printing sheets.
A method of obtaining colored toner particles by the above-mentioned polymerization granulation technique is disclosed in Japanese patent application (OPI) No. 52588/78. This method comprises adding dropwise a solution comprising a monomer which is soluble in a non-polar solvent but becomes insoluble upon polymerization, a dye which is soluble in a monomer and insoluble in the non-polar solvent, and a polymerization initiator to a non-polar solvent containing a charge control agent, etc. to effect a polymerization reaction. According to this method, the dropwise addition of the solution simultaneously induces dissolving of the monomer in the non-polar solvent to thereby form fine crystals of the dye. Thereafter, the monomer is polymerized to obtain a liquid developer in which the resulting polymer is adsorbed onto the fine crystals of the dye.
However, the above-described method involves a serious problem as set forth below. A number of dyes have a chemical structure in which a radical, e.g., a phenolic hydroxyl group, a phenylamino group, or a nitrophenyl group, can stably exist. Most of the dyes having such a chemical structure function as a substance which inhibits radical polymerization of monomers, and are known as radical polymerization inhibitors. Therefore, presence of such dyes in the system employed for radical polymerization of monomers as described above interferes with the polymerization reaction, resulting in an entire failure of, in extreme cases, adsorption of a polymer onto the fine crystals of the dye. Accordingly, the dyes which can be used in this method should satisfy both requirements that they do not act as a radical polymerization inhibitor and that they are insoluble in non-polar solvents but soluble in a resinous monomer to be polymerized. Hence, the choice of materials for the dyes and monomers to be used is quite limited, and many dyes virtually cannot serve the purposes of this method. Further, dispersion stability of the particles prepared by these polymerization granulation methods are too poor to be applied to practical use.